CHARLESTON, S.C. — A jury signaled here on Friday that it was within a single vote of convicting a white former police officer in the fatal shooting of an unarmed black man.

But confusion reigned in the courtroom as the jury wavered about whether it was hopelessly deadlocked about the killing, which was recorded on video and became a symbol of the nation’s heated debate about race and policing.

Over the misgivings of a defense lawyer, Judge Clifton B. Newman, who presided over a four-week trial, did not declare a mistrial in the case of the former officer, Michael T. Slager, who shot and killed Walter L. Scott in North Charleston, S.C., on April 4, 2015. Jurors had been deliberating for about 48 hours and, by day’s end, had decided to resume their discussions on Monday.

Yet Friday’s proceedings, before a crowded courtroom that alternated between focused and flummoxed, were a remarkable display of the divisions in one of the country’s most closely watched cases. It has been seen as a bellwether for whether video evidence could lead to a rare conviction of a law enforcement officer in an on-duty killing.

“I cannot in good conscience consider a guilty verdict,” an unidentified juror wrote in a letter to Judge Newman, who read it aloud in court. The juror added that the panel “may never reach a unanimous decision.”

In a separate note, the jury’s foreman, the sole black member of the panel, appeared curt, writing, “It’s just one juror that has the issues.”

It was not clear whether the panel favored a conviction for murder, which could lead to a sentence of life in prison, or voluntary manslaughter, which carries a sentence of two to 30 years. Mr. Slager was indicted on a murder charge, and Judge Newman allowed jurors this week to consider the lesser offense of manslaughter.

But the dueling messages — and the shifting positions from jurors about whether additional talks or instructions might yield a verdict — were a stunning turn in a case that began nearly 20 months ago, when Mr. Scott was shot and killed after fleeing from a traffic stop for a broken taillight.

Mr. Slager, whom North Charleston fired after the shooting, testified that he had become involved in a physical struggle with Mr. Scott over his Taser device, leaving him in “total fear” and causing him to open fire in a lot behind a pawnshop in the city of about 108,000.

“He didn’t shoot him because of the brake light,” Andrew J. Savage III, Mr. Slager’s lead defense lawyer, said of the former officer during his closing argument. “He shot him in fear of his life.”

It was an account that, at first, local officials believed. But within days, a pedestrian’s cellphone video became public and showed a gruesome shooting that shook this region and reverberated around the world: Mr. Slager firing eight rounds toward Mr. Scott, who was running away and was at least 17 feet from Mr. Slager.

Prosecutors argued that Mr. Slager had “malice aforethought” when he shot Mr. Scott, and that he had staged evidence in a frantic effort to justify the shooting, the first time he had fired his service weapon while on patrol.

“The badge is supposed to be a shield, not a sword,” Scarlett A. Wilson, the chief prosecutor for Charleston County, said during her final presentation to the jury, when she argued that jurors could not acquit Mr. Slager “knowing these facts and South Carolina law.”

For a time on Friday, the proceedings appeared poised to end in a mistrial, an outcome that would have been similar to the results of other cases involving accusations of police misconduct. A judge in Baltimore declared a mistrial in December when a jury could not reach a decision in the case of an officer charged in connection with the death of Freddie Gray, and a Cincinnati jury deadlocked last month during its deliberations about a police officer who had shot and killed an unarmed black motorist.

Shortly after lunch on Friday, the jury sent a note indicating it was at an impasse, prompting Judge Newman to deliver what is known as an Allen charge, a set of legal instructions intended to break deadlocks during deliberations.

“You have a duty to make every reasonable effort to reach a unanimous verdict,” the judge told the jury of seven men and five women as he read the Allen charge, named for the 1896 Supreme Court case that enshrined such guidance.

“The majority should consider the minority’s position, and the minority should consider the majority’s position,” Judge Newman told the jurors before ordering them to resume their deliberations in hopes that they would come to one of three conclusions: a conviction for murder, a conviction for manslaughter or an acquittal.

The outward orderliness of the trial collapsed within hours, when the jury sent more notes that revealed details about the deliberations. The jurors veered from hopelessly deadlocked to willing to talk more, rattling the courtroom, which was filled with members of the Scott and Slager families, as well as onlookers who had gathered, they thought, to see an end to the trial.

But Judge Newman was reluctant to end the proceedings, even as Mr. Savage protested that continued deliberations could be “extremely coercive.”

The jury, after sending another note announcing that it was exhausted, left the courthouse after nightfall. And Mr. Savage, a longtime defense lawyer in Charleston, seemed bewildered as he walked through a crowd of cameras after the court adjourned.

“Let’s just wait till Monday, see what happens Monday and then we’ll come in after that,” he said.

A lawyer for Mr. Scott’s family, L. Chris Stewart, offered a similar reflection as he left the courthouse.

A version of this article appears in print on , on Page A10 of the New York edition with the headline: Jury Appears One Vote Shy of Convicting Ex-Officer in Fatal Shooting. Order Reprints | Today’s Paper | Subscribe